Typically, the discernment and evaluation of odours is performed by the olfactory sense of the human being. By this method, it must be considered that different persons or panels have different olfactory sensitivities and the olfactory sense of a panel may vary depending on such mitigating factors as mood, alertness, health, etc. of the panel on the day of the test. Therefore, to obtain an objective result with high systematic accuracy, it is necessary to gather an adequate number of persons and to conduct the test under an adequately uniform environmental condition. Typically, current methods for assessing odour include the use of panels of untrained people to qualify the odour characteristics (tone, intensity and overall perceived air quality) as per VDI 3882/2, VDI 3882/1 and Fanger's decipol calculation method (Fanger, 1988, “Introduction of the olf and the decipol units to quantify air pollution perceived by humans indoors and outdoors”, Energy and Buildings, 12, 1–6, the disclosure of which is incorporated herein by reference).
However, these methods have not been used for overall odour quantification, and thus using the existing methods of analysing the results, comparisons of odour removal performance of equipments used to remove unpleasant odours from an airstream are not possible.
In aircraft cockpits and cabins unpleasant odours leading to discomfort of the crew and passengers often originate from so-called VOCs (Volatile Organic Compounds) that lead to these odours. VOC contamination in the aircraft cabin inlet airstream often stem from ground operations of the aircraft, such as push back, taxiing, etc., by which the airport air may be contaminated by the engine exhaust fumes, fuel fumes and other air borne contaminants. These contaminants are ingested by engines and Auxiliary Power Units and may lead to contamination of the bleed air supply.
There are several recognised methods by which the engine exhaust fumes and fuel fumes can be ingested into the aircraft cockpit and cabin, for example during ground operations when taxiing behind another aircraft or in the queue for take off VOC-containing airstreams enter the aircraft cockpit and cabin. Furthermore, certain wind conditions at engine shut down if the Auxiliary Power Unit has already been started or during ground operations the exhaust fumes of the ground service vehicles may also lead to an ingestion of contaminated air by the Auxiliary Power Unit.
Several different types of equipment are currently proposed to remove VOC that lead to these unpleasant odours in the cabin or cockpit air. However, while there are means by which the efficiency in removing these VOCs can be measured, there is no recognised method by which the efficiency in reducing the odour due to the VOCs can be quantified. It is often the case that strong and unpleasant odours may be attributed to very small amounts of some specific VOCs, whereas larger amounts of others do not significantly contribute to unpleasant odours. At present, it is impossible to accurately assess the claims of available technology in the field of odour reduction.